The best translation is not with ένα, but with έναν. If the exercise was a tile exercise (where you have to pick the words), this is not something we can control. The tiles are picked randomly from the translations, so they could be from any of them.

(Ένα probably ended up in the best translations because the original Greek sentence had a mistake with the article.)

So, to sum up in accusative sentence ένα and ένας (respectively neutral and masculine) gain an N, while μια doesn’t change.
Also when έναν is before a word that began with a consonant the last N can be dropped off.